1. Field of Endeavor
The present invention relates to a burner for operating a heat generator, wherein such a burner has a swirler for a combustion air flow, and also means for injecting at least one fuel into the combustion air flow. Downstream of the swirler, a mixing path is arranged, and in the region radially outside the discharge opening of the mixing path of the burner there is at least one nozzle for feeding liquid pilot fuel. Furthermore, the present invention relates to a method for operating such a burner.
2. Brief Description of the Related Art
Premix burners, as proposed for example in EP 0 321 809 B1, are burners in which a fuel, gaseous or liquid, is first mixed with the combustion air and after this mixing process is combusted in the flame. In the case of the type of such a premix burner which is proposed in EP 0 321 809 B1, a plurality of conical wall elements are provided, wherein these wall elements are arranged in an offset manner to each other in such a way that inlet slots for the combustion air into the interior of the burner are formed between them. In this region, therefore, a swirl is generated, and the swirled flow which is formed therein is then transferred into a mixing path. In such a burner both liquid as well as gaseous fuels can be combusted, wherein the former are preferably fed on the axis of the burner via a fuel lance, and the latter are fed in the region of the inlet slots, typically via a multiplicity of exit orifices which are arranged in series. Such burners are characterized by an outstanding stability of the flame and also by excellent pollutant values (low NOx values) and efficient heat generation.
A further improvement of such a construction is described, for example, in documents EP 0 704 657 B1 and EP 0 780 629 B1. In this case, a mixing path is also arranged downstream of the swirler formed by the conical wall elements, and specific transfer passages, which ensure an ideal transfer of the flow which is formed in the swirler into the mixing path, are provided at the inlet of this mixing path.
In the case of such burners, the fact that they have the tendency to become unstable, if for example they are controlled under low-load conditions or under transient conditions with a low fuel supply, is problematical. This is because, inter alia, such burners ideally have to be operated close to the lean quenching limit in order to have the aforementioned advantages. If the fuel supply is lowered below a critical value, then quench pulsations can occur, that is to say a quenching of the flame can be caused as a result of oscillations in the combustion chamber (so-called thermoacoustic instabilities).
In order to avoid such problems, a so-called pilot mode had been proposed on a number of occasions, that is to say an operating mode in which special additional fuel nozzles, which can be controlled under such low-load conditions or in the case of transient conditions, are arranged at suitable places of the burner or in the combustion chamber.
So, for example, EP 0 994 300 B1 describes the possibility of injecting gaseous pilot fuel in the case of a burner of the type as is described in EP 0 704 657 B1 or in EP 0 780 629 B1, this virtually being at the front edge of the mixing path, wherein swirl generators are additionally arranged in the region of the outlet of this pilot fuel. As a result of the vortex plaits which are created on the swirl generators, an increased mixing of the combustion air with the pilot fuel is brought about, and higher stability of the combustion process and lower pollutant values accordingly. As a result, the effect can be achieved of the operating range of such a burner being able to be extended to the bottom end with constant pollutant values.
Another possibility for feeding gaseous pilot fuel is described in EP 0 931 980 B1, wherein the gas in a discharge ring of the burner, after mixing with combustion air, is ignited by an ignition unit and injected into the combustion chamber.
While the aforementioned systems relate exclusively to the feed of gaseous pilot fuel, EP-A-1 389 713 in addition also describes the feed of liquid pilot fuel, after mixing with combustion air, into the combustion chamber very close to the discharge opening of the burner, this feed also being on the front outlet edge which faces the combustion chamber and specifically from a conical flank of the discharge ring which is bevelled outwards and towards the burner rear wall. Since liquid fuels on the one hand as a rule are more easily combustible, the pilot mode can also be maintained beyond the partial load, and since when feeding liquid fuel after shutting down it is not mandatory to be purged with air, this is of great advantage.
In order to get a grip on the problem of the excessive heat which occurs in the region of the outlet edge, the feed via fuel pipes with discharge openings arranged at their ends is described in EP-A-1 389 713, wherein the discharge openings do not lead directly into the combustion chamber, but, rather, lead into an encompassing cavity in the discharge ring which is arranged in the region of the outlet edge directly next to the burner opening and which is purged with combustion air and has holes which are arranged above the discharge openings or nozzles respectively and through which the liquid fuel can discharge into the combustion chamber from the said flanks. In order to be able to ensure the stability of the pilot flame, the fuel is introduced into the combustion chamber in a jet which is arranged in a plane which includes the axis of the burner. It is specified that the jet with the axis of the burner forms an angle within the range of 15 to 600. The discharge openings are indeed exposed to circumflow on their surface which faces the combustion chamber by the combustion air which is fed in the ring, but the cooling still has optimization requirements because an uneven distribution of the air through the air ring occurs, and consequently an uneven cooling. There is also the fact that the cold fuel in this case gives rise to a high temperature gradient which leads to high stresses.
For better mixing of the liquid fuel with the combustion air, it is necessary, moreover, to arrange swirl generators for the liquid fuel in the feed line upstream of the nozzle which is arranged at the discharge opening. It is specifically disclosed that, for example, a perforated plate, with at least two holes for the generation of such turbulence and which is installed in the pipe cross sections of the feed pipe, can be used.
Since the pilot nozzle for the liquid fuel is integrated in the discharge ring in a fixed manner, and the same purging air is used as for the gas pilot, there is a further disadvantage of the solution which is known from EP-A-1 389 713, in that in case of damage, the entire burner head has to be exchanged which gives rise to high costs.